1. Field of the Invention
This invention relates to a video signal reproducing apparatus having means for correcting the so-called skew condition which results from discontinuation of a horizontal synchronizing signal occurring during reproduction of a video signal from a recording medium.
2. Description of the Related Art
In repeatedly reproducing one field portion of a video signal recorded in a recording track with a video signal recording and reproducing apparatus, a reproduced horizontal synchronizing signal becomes discontinuous at the start-and-end point of the record. As a result, the upper part of an image plane thus obtained becomes obliquely distorted. This is called a skew phenomenon. For example, referring to FIG. 1 of accompanying drawings, when one field portion of a video signal recorded in a circular track T2 on a disc-shaped recording medium is to be repeatedly reproduced, a horizontal synchronizing signal included in the record comes to discontinue for a period of 1/2 H (H representing a horizontal scanning period) at a start-and-end point 1a. The illustration includes a PG pin 1d which is made of a magnetic material and is arranged to indicate a position corresponding to the start-and-end point 1a. The horizontal synchronizing signal is, in general, supplied to an AFC circuit 2 which is, for example, arranged as shown in FIG. 2 before it is supplied to a horizontal drive circuit (not shown) within an image receiver. The AFC circuit 2 consists of a voltage controlled oscillator (hereinafter referred to as VCO) 2a; a trapezoidal wave generating circuit 2b; and a sample-and-hold circuit 2c. The reproduced horizontal synchronizing signal taken out from the recording medium 1 is supplied to the sample-and-hold circuit 2c of the AFC circuit 2. Then, a horizontal synchronizing signal which is to be used at the horizontal drive circuit is taken out from the VCO 2a.
FIG. 3 shows the wave form of the signal produced from the trapezoidal wave generating circuit 2b of the AFC circuit 2. In FIG. 3, a point A indicates a point at which the signal is sampled and held by the sample-and-hold circuit 2c. The point A is located at the middle point of a linear line part in the output wave form. The period t of the linear part is shorter than one H period. Therefore, a period of time t1 between commencement of scanning for one H period and the point A is in a relation t1&lt;1/2 H to the period H. Therefore, when the horizontal synchronizing signal in which the 1/2 H discontinuity arises as mentioned above comes to the sample-and-hold circuit 2c, a certain period of time is required after deviation from synchronism and before resumption of the synchronism at the AFC circuit 2. Therefore, picture quality is greatly deteriorated by the occurrence of the skew condition having an oblique distortion in the upper part of the image reproduced on a Braun tube by the signal produced from the VCO 2a before resumption of synchronism.
To avoid the adverse effect of the deviation from synchronism due to the 1/2 H jump of the reproduced horizontal synchronizing signal, a method of compensating for the above-stated discontinuity of the horizontal synchronizing signal has been known. In this method, an ultrasonic delay line is used for obtaining a H/2 delayed signal by having the field signal of one and the same track allowed either to pass through the H/2 delay line or not to pass through the delay line. The H/2 delayed signal and the signal not delayed are then alternately produced via switching means to compensate for the discontinuity. This method, however, necessitates use of means for effecting the H/2 delay and has presented a problem that it results in a complex structural arrangement.
In view of this, a device which is capable of preventing the skew condition with a simple structural arrangement is disclosed in Japanese Laid-Open Patent Application No. Sho 49-127518. In the case of this device, the scanning period including the H/2 phase deviation which inevitably exists in each field frame unit of a reproduced video signal is removed by gating. During the gate removing scanning period, an oscillator which produces pulses at a period somewhat longer than one H period is allowed to oscillate by itself. The skew condition is prevented by using the oscillation output of the oscillator as a horizontal synchronizing signal and by replacing the period including the gate removed H/2 phase deviation with a dummy horizontal synchronizing signal which equally divides the H/2 deviation including period. In the case of such a device, the free running oscillation period of the above-stated oscillator must be accurately maintained. However, the oscillation period is apt to be affected by the drift of temperature, etc. In the event of even an error of only several percent due to such a change in temperature, the horizontal synchronizing signal becomes discontinuous upon cancellation of the gate removing state. In that event, not only it is impossible to attain any skew correcting effect but sometimes a contrary effect might result.